Low Temperature Cofired Ceramic (LTCC) materials are widely used as multilayer ceramic substrate materials. If a low temperature cofired ceramic is used, when a relatively inexpensive and low specific resistant low-melting point metal such as silver or copper is used as an electrode material such as an internal electrode, a surface electrode, or a via electrode, the low temperature cofired ceramic and the low-melting point metal can be co-fired. Therefore, a multilayer ceramic substrate excellent in high-frequency characteristics can be manufactured at low cost.
For example, Patent Document 1 (Japanese Patent No. 5,533,674) discloses a low temperature cofired ceramic material containing a main component, containing 48 to 75% by weight of SiO2, 20 to 40% by weight of BaO, and 5 to 20% by weight of Al2O3, 2.5 to 5.5 parts by weight of MnO as a sintering aid component with respect to 100 parts by weight of the main component, and other auxiliary components.
As described above, the low temperature cofired ceramic material has an advantage that it can be co-fired with a low melting point metal such as silver or copper; however, it also has a problem that a substrate shrinks greatly in the planar direction during firing.
In view of this, in the method for manufacturing a multilayer ceramic substrate that is disclosed in Patent Document 2 (Japanese Patent No. 5,024,064), constraining layers are inserted between ceramic layers each formed of a low temperature cofired ceramic material to form a laminate, and shrinkage in the planar direction of a substrate is suppressed by firing the laminate. Patent Document 2 discloses that the constraining layer uses, for example, Al2O3 (alumina), ZrO2 (zirconia), or the like (see paragraph (0035)). Patent Document 2 further discloses that the constraining layer may contain a glass component (see paragraph (0058)). The constraining layer remains in the laminate even after the multilayer ceramic substrate is completed.
According to the method for manufacturing a multilayer ceramic substrate disclosed in Patent Document 2, shrinkage in the planar direction of the substrate can be suppressed. However, the manufactured multilayer ceramic substrate has a structure in which ceramic layers and constraining layers formed of two types of materials exhibiting different shrinkage behaviors are stacked over the entire thickness direction of the substrate, so that there have been problems that an amount of warp of the substrate is large, flexural strength is low, and electrode bonding strength between the substrate and a surface electrode is low.
Thus, in order to solve this problem, Patent Document 3 (Japanese Patent Application Laid-Open No. 10-84056) discloses a method for manufacturing a multilayer ceramic substrate in which, while a laminate itself constituting the multilayer ceramic substrate has a multilayer structure of low temperature cofired ceramic layers, the laminate is fired so as to be vertically sandwiched between constraining layers with pressure. Patent Document 3 discloses that the constraining layer uses, for example, MgO, Al2O3, ZrO2, or the like (see paragraph (0020)). The constraining layer is removed from a surface of the multilayer ceramic substrate after firing.
The multilayer ceramic substrate manufactured by the method disclosed in Patent Document 3 solves the problem of the multilayer ceramic substrate manufactured by the method disclosed in Patent Document 2. That is, according to the method disclosed in Patent Document 3, a ceramic portion of the substrate is formed of one kind of low temperature cofired ceramic material. Thus, it is possible to solve the problems of the method disclosed in Patent Document 2, that the amount of warp is large, the flexural strength is low, and the electrode bonding strength between the substrate and the surface electrode is low, due to the fact that the substrate is formed by the low temperature cofired ceramic layer and the constraining layer exhibiting different shrinkage behaviors.
Patent Document 1: Japanese Patent No. 5,533,674
Patent Document 2: Japanese Patent No. 5,024,064
Patent Document 3: Japanese Patent Application Laid-Open No. 10-84056